As is known, automatic cold-junction referencing of thermocouples is achieved electrically by creating an electrical signal which has the same voltage versus temperature characteristic as the thermocouple cold-junction signal has over a range of ambient temperatures. This signal is then added directly to the incoming thermocouple signal to compensate for changes in ambient temperature. The technique has been used for a number of years, is quite accurate, and widely accepted. It is used in commercially-available thermocouple-feedback temperature controllers. However, these controllers cannot be programmed with a voltage source such as computer-based systems with digital-to-analog converters, except under the very restrictive condition that the thermocouple be isolated from ground.
Two approaches to the problem of measuring the output of a grounded thermocouple are taken in modern temperature controllers. In one approach, the entire controller is floated above electrical ground, permitting the thermocouple to be grounded. This requires a high degree of isolation from ground and from the input alternating current power source. In the second approach, a differential amplifier is used to isolate the thermocouple from all other grounds. However, in this device, the cold-junction compensation and set point versus feedback summing is achieved prior to amplification. This approach requires an additional highly isolated power supply to provide a source of voltage to the set point and cold-junction compensating circuits. Neither of these approaches will accept a single-ended voltage command when a grounded thermocouple is used.
In the past, various techniques have been employed in an attempt to solve the problem of making a temperature controller which is compatible with a computer-based command generator. One method has been to isolate all thermocouples from ground. This, however, is not a very desirable approach since the best thermal contact is achieved by spot-welding or mechanically fastening the thermocouple to the heated part. In another attempt to solve the problem, voltage programming is avoided, which requires the use of an isolated device such as a manual- or motor-driven potentiometer.